This invention relates, in general, to subsurface formation testing apparatus, and more particularly to methods and apparatus for providing real time simultaneous measurements of pressure and temperature of fluid samples from subsurface earth formations traversed by a borehole.
The sampling of fluids contained in subsurface earth formations provides a method of testing formation zones of possible interest by recovering a sample of any formation fluids present for later analysis at the earth's surface while causing a minimum of damage to the tested formations. Thus, the formation sampler is essentially a point test of the possible producibility of subsurface earth formations. Additionally, a continuous record of the sequence of events during the test is made at the surface. From this record valuable formation pressure and permeability data can be obtained for formation reservoir analysis.
Early formation fluid sampling instruments, such as the one described in U.S. Pat. No. 2,674,313, were not fully successful as a commercial service because they were limited to a single test on each trip into the borehole. Later instruments were suitable for multiple testing; however, the success of these testers depended to some extent on the characteristics of the particular formations to be tested. For example, where earth formations were unconsolidated a different sampling apparatus was required than in the case of consolidated formations.
One major problem which has hampered the reliable testing of subsurface earth formations has been in obtaining an accurate and repeatable measurement of the subsurface formation pressure. The typical subsurface formation testing instrument employed only a pressure sensor. Temperature corrections to the pressure measurement comprised a manual correction based on a temperature measurement derived at a spaced-apart location within the wellbore. A typical system utilizes a thermometer located within a cable head which records the maximum wellbore temperature. When the sampling instrument is returned to the surface the recorded temperature measurement is used to correct the pressure measurement.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing method and apparatus for providing real time, simultaneous measurements of the pressure and the temperature of fluid samples from subsurface earth formations and methods and apparatus for correcting the pressure measurements for the effects of temperature.